Hi!
Is it possible to mask the browser agent - to prevent websites from discovering that I'm on a cell phone?
Something like the agent switcher add-on for Firefox? http://chrispederick.com/work/user-agent-switcher/
Possible to mask the browser agent?
Not as a phone-user without hacking the firmware (and how it is is possible or not depends on which phone).
Easest way is to use a proxy server that does it for you. Easiest way to do that, is to use a browser that already is integrated with the browser developer's proxy (e.g., Opera Mini, BOLT, Skyfire, TeaShark, andwhatnot).